Hollow Bore Absolute Encoder by 221 Robotic Systems

Wanted to drop a line and let the FRC community know that we’re launching a new sensor product for 2020!

We’ve been working on hollow bore sensor concepts for about three years. We finally found a form-factor and technology that we think is a great fit for FRC.

This particular version of our sensor was conceived for swerve steering feedback but works in all kinds of applications.

It is thin, easy to mount and the large magnet bore leaves a lot of room for the designer to integrate as they see fit.

The magnet can be sensed from either side of the board as well!

This class of sensor is designed to compete with and/or replace a US Digital MA3. It generates a 3.3v or 5v analog output so you can use it just like a potentiometer.

It plugs directly into an SRX and can be used just like a CTRE Mag Encoder for closed-loop control.

The magnet is made from a durable polymer material. It is designed to be glued in place by the user.

We’re in production now and plan to have these ready in November.

Lamprey Absolute Encoder
-10-bit resolution
-3.3v or 5v analog
-5x2 header can directly interface with Talon SRX
-Large bore, 1.23" max.
-Mount hole on 2 in. diameter…fits directly to many popular FRC components.
-User-settable zero position

MSRP $41.25 with magnet

Lamprey LP-001-1 Layout Drawing

Lamprey Kit with Magnet.STEP

Lamprey LP-001-1 Board Render


AM Dual Motor Gearbox mounting

Versa Planetary Mount

Inside an MK2


221 is back… what an (re)entrance. :smile:

This. Is. Fantastic.

Getting a 1:1 absolute encoder on a large dead axle was always a slight pain. Not anymore.

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As someone who nose-mounted a nonzero number of AM Sports over the last couple years, I’m curious how well this will play with some of the common methods (the new AM flange bracket, REV’s motor plates, etc).

But if it means a no-printing-no-machining way to get an encoder going on our stuff, I may just be able to suck it up. :slight_smile:

Edit to add: What does the magnet get glued to?

Thank you Anthony! I remember going down this rabbit hole last year and finding nothing in the “reasonable” price range so super happy to see this exist now.

Looks like to the axle, or to a bushing if you need to expand your shaft’s radius. 3dp would probably be the easiest way to make a bushing for whatever you are fitting it to.

Can you provide the correct dimension range for the distance between either side of the board and the magnet for accurate sensor reading?

We as a team have been talking about something similar to this for swerve, might be able to make this work with our design.

EDIT: After looking at our application I wonder if either of the red components on the board could be flipped? Having one side of the board as flat and low profile as possible would help with packaging in most applications. @ajlapp is this something that could be done?

Also would have a bit of concern with an open exposed board like this in different applications that it could be more easily damaged without some kind of casing.

We had talked about using an induction style sensor that would allow for a 360* board and a 180* antenna. This would allow for much easier application to a swerve setup, as often it is more difficult to package and mount a 360* ring on the module itself as there are gears in the way.


We have the same thought regarding integrating into our swerve configuration.

We will have a version without the secondary push button so the entire bottom is flat.

The board is conformal coated but will be exposed. You’ll have to take care to not crush it.

We could have conceived a case but that drives cost.

With 3DP options so readily available these days we decided to leave it to the end-user.

This is the same approach we took with hub design. We started down the path of making one but decided to leave it open to the user.

How about a magnet in the ~4" or so diam range?

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The biggest we’ve successfully pulled off is 2.5” OD.

Based on everything I know it might not be feasible…if it can be done it would be very expensive.

Will you have just the magnets available for purchase so that the board can be re-used if the magnet is permanently installed somewhere?



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Could the magnet ring be machined slightly (i.e.: open the bore to 1.25" or reduce the OD to 1.50") to fit an existing piece of rotating stock?


great concept.


  • How well does the sensor manage shaft misalignment (shafts offset or not coaxial)?

  • (Reiterating Nick’s first Question) What is the entire range of operation (axially) between the sensor and the magnet? Can the sensor change range distance on the fly without losing position?

It might be able to be cut but it will probably damage it.

The ID is sized to pilot on the flange of a 1/2” bearing.

The OD was the closest fractional size that didn’t interfere with the mounting bolt heads and also magnetized properly.

I will get some additional info for Skyehawk…the short answer is you can calibrate concentricity issues out and it is fairly tolerant to air gap adjustments on the fly.


The magnet range is 0.090"-0.500" from the face of the sensor IC. Closer than 0.090" will saturate and cause dead spots in the rotation. Great than 0.500" generates a very weak signal.

You can translate the magnet through this range dynamically. You due tend to get some noise in the signal during the translation…maybe ±1 degree.

The magnet can be sensed through non-ferrous materials so it can be placed on either side of the PCB.

I updated the STEP models to reflect this…the original showed a 0.050" gap which is a bit too close.

Sensor Board with Magnet.STEP

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Darn, thought I was able to squeeze it into ours. Not sure I can get away with another 0.040".

Non-metallic/non-conductive? Or are you saying I could actually hide the PCB inside an aluminum tube?

I’ve never placed the magnet completely inside an aluminum tube but it definitely works fine through a plate of aluminum.

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